The present disclosure relates to a method for dicing substrates, and particularly, to a method of dicing substrate with solder bumps and an underfill layer thereupon.
Dicing is a process in which a chip-containing substrate is cut into individual dies. The chip-containing substrate typically includes a vertical stack of a semiconductor substrate including semiconductor devices and a metal interconnect structure-containing layer including dielectric material layers and metal interconnect structures embedded therein. Each die typically includes a semiconductor chip, and can be subsequently bonded with another substrate in a bonding process such as flip chip assembly. A diamond blade dicing process has been used in the industry for singulation of dies from the chip-containing substrate.
A chip-containing substrate includes a semiconductor device layer and a metal interconnect layer that includes metal interconnect structures embedded in a dielectric material layer. Recently, low dielectric constant (low-k) dielectric materials having a dielectric constant less than 3.9 (the dielectric constant of silicon oxide) and porous ultra low-k dielectric materials having a dielectric constant less than 2.8 layers have been employed as the dielectric material embedding the metal interconnect structures. Because such low-k and ultra low-k dielectric materials are prone to structural damage during laser dicing, formation of grooves in the dielectric material layer embedding metal interconnect structures has been recently proposed. Specifically, a laser grooving process can be performed first on the active side of the chip-containing substrate, i.e., the side at which the dielectric material layer embedding metal interconnect structures are located. Low-k and/or ultra low-k dielectric material layers and metallic structures in the dicing channels are ablated by a laser beam. The metallic structures in the dicing channels typically include test structures and alignment structures, and are referred to as kerf structures. Full singulation of the dies is accomplished by cutting through the remaining portion of the chip-containing substrate, i.e., the semiconductor substrate, in the dicing streets with a diamond saw process.
The two step die singulation process of laser ablation followed by diamond saw dicing requires that the dicing channels are visible for laser grooving. The two step die singulation process can be employed for conventional wafers, i.e., chip-containing substrates, which do not employ a wafer level underfill (WLU) material. As used herein, a “wafer level underfill,” an “underfill,” an “underfill material,” or “WLU” refers to a underfill material that is applied on a wafer surface or over an array of solder bumps on a wafer, i.e., a chip-containing substrate prior to singulation of the dies therein. However, underfill materials at a thickness comparable with the height of solder bumps (balls), which have a diameter on the order of 10˜100 microns, are optically opaque. Thus, the two step die singulation process is not compatible with application of an underfill material (such as wafer level underfill (WLU) as known in the art) prior to singulation.
Specifically, when a wafer level underfill is applied on a wafer, it is very difficult to use the normally used laser grooving method which is critical to prevent damage induced on brittle low K and ultra low K dielectric layers during the wafer sawing process and subsequent solder reflow chip join process for the following reasons.
Firstly, the laser grooving process needs very precise control in positioning of the laser beam to avoid hitting active features on closely spaced die. The WLU is coated almost the same height as the interconnection solder bumps or slightly thicker than the solder bump height, so even though the transparency of B-stage cured WLU can be good enough to see the solder bumps through a thickness of a few microns, the dicing marks are not visible through the thickness on the order of 10˜100 microns. Thus, once the WLU is pre-applied on the wafer, the dicing marks on the active surface of wafer, i.e., on the surface of the semiconductor substrate, are not visible.
Secondly, compared to blade dicing which uses cooling wafer during wafer sawing process, laser grooving does not use any cooling source and the temperature during laser grooving reaches a localized temperature of at least 1,420 degrees Celsius (melting point of Si). Therefore, laser grooving after a WLU coating has been applied to the wafer causes melting and curing of WLU material near the ablation groove edge. The high temperature of laser ablation process results in a very wide heat affected zone, which typically reaches the solder bump areas. Melting and curing of the WLU material necessarily occurs adjacent to the ablation path. The altered physical state of the WLU material affects the flow and curing of WLU during the subsequent flip chip assembly process.
Therefore, there is a need for a singulation method that is compatible with a substrate having an array of solder bumps and a wafer level underfill (WLU) material thereupon.